Seismic surveying is used for identifying subterranean elements, such as hydrocarbon reservoirs, fresh water aquifers, gas injection reservoirs, and so forth. In performing seismic surveying, seismic sources and seismic receivers can be placed at various locations on an earth surface (e.g., a land surface or a sea floor), or even in a wellbore, with the seismic sources activated to generate seismic waves. Examples of seismic sources include explosives, air guns, acoustic vibrators, or other sources that generate seismic waves.
Some of the seismic waves generated by a seismic source travel into a subterranean structure, with a portion of the seismic waves reflected back to the surface (earth surface, sea surface or sea floor, or wellbore surface) for receipt by seismic receivers (e.g., geophones, hydrophones, etc.). These seismic receivers produce signals that represent detected seismic waves. Signals from the seismic receivers are processed to yield information about the content and characteristics of the subterranean structure.
In seismic surveying, coherent noise can contaminate seismic signals detected by seismic receivers. “Coherent noise” refers to seismic source-induced noise that mainly includes horizontally-traveling waves confined in the so called near surface (a weathered layer adjacent the surface). The horizontally-traveling waves travel from the seismic source to seismic receivers. Generally, the coherent noise waves travel at a relatively low velocity when compared to waves associated with useful signals (signals reflected from the subterranean structure that is being surveyed).
Conventional filtering techniques separate useful signals from coherent noise based on the velocity difference between the useful signal and the noise. For example, a velocity filter can be used to remove signal components having energy within a particular range of velocities. Examples of velocity filters include an f-k filter, tau-p filter, and so forth. However, a shot gather having a three-dimensional (3D) acquisition geometry prevents straightforward application of certain types of velocity filters. A “shot gather” refers to seismic traces collected in response to multiple shots of multiple sources (where the multiple sources can actually be a single source moved across different positions). Moreover, near-surface wave phenomena typically have multiple modes of propagation, which can be superimposed onto the useful signal. Conventional noise filtering techniques may not be accurate when multiple modes of noise are present.